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Customers who preordered the eagerly anticipated sixth installment of the "Harry Potter" saga on Amazon.com (http://amazon.com) and walmart.com (http://walmart.com) were in for a shock this week when the retailers mistakenly e-mailed them to say their books might arrive later than expected.

"Harry Potter and the Half-Blood Prince" is slated for release early on Saturday, and both Web sites had promised anxious customers that their books would arrive that day. The U.S. publisher of J.K. Rowling's book said some 10.8 million copies were printed.

Wal-Mart Stores Inc. rushed to calm shoppers' worries and put a recorded message on walmart.com's toll-free customer service number assuring customers that the books would arrive on July 16 as promised.

"If you are calling in reference to the Harry Potter release notification that you received on July 12 that indicated the release date had been changed to July 20, please be advised that the e-mail was sent in error," the message says.

Amazon.com e-mailed customers to say its notice that books might be delayed "was sent to you in error."

"We sincerely apologize for the concern we caused with this incorrect message, and hope you will be thrilled to know that we've begun preparing your order for shipment and we are confident that it will be delivered on Saturday, July 16," the e-mail to customers reads.

Amazon.com spokeswoman Patricia Smith said a few thousand customers mistakenly received the notices.

"It was a complete goof on our end," Smith said. "I don't know if it was human error or computer error, but the bottom line is, it was an error."

"I got one of those e-mails," reads one post on fan Web site mugglenet.com. "I'm getting a second copy at midnight just in case, because if it doesn't arrive I'm canceling my transaction. I'm still furious."